August 2014
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The Importance of Mechanical Details

The devil is in the details

Steven R Calabrese

Steven R. Calabrese
Control Engineering Corp.

Contributing Editor


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Whenever I’m bidding a plan/spec project, the first thing I do is print out the mechanical plans. Which is easier said than done if there are dozens and dozens of them! The point is, some of the plans are, shall we say, less important than others. For instance, if demolition plans are included in the mechanical set, and I’m excluding anything to do with demo, then I may choose not to print out these particular sheets in full-size format. Other plans, such as ventilation schedules, may have little to no significance to what I need to know in order to accurately quote the project from a controls perspective.

Mechanical details typically take up one or two sheets of the mechanical plans of any given plan/spec job. My first instinct is always to ignore these sheets. Get to the heart of the project by studying the floor plans, piping/ventilation schematics, and equipment schedules. However my better judgment always leads me back to these drawings, as I’ve been burned in the past by disregarding them as “mechanical” information with no relevance to controls.

So the purpose of this column is to explore these details, in a general manner, and point out some items that relate to the temperature controls scope of any given project. As we will see, there are little things “hidden” in these details, that may not be included anywhere else in the plans and specifications, and they need to be accounted for. Moreover, there may even be inconsistencies between these details and information included elsewhere in the contract documents. In any case, these things need to be addressed, accounted for, quantified, clarified, etc. In the end, the consulting engineer will hold you to the information included in these details, so it’s best to be aware of their importance, and to give them equal consideration in the contract document review process.

Coil Details

You’ve seen them. An isometric depiction of a hot or chilled water coil, with piping and all the amenities. The purpose of these details is to show what simply cannot be shown on the floor plan representation of the coil, which is typically internal to a piece of equipment or some ductwork. The main thing for the controls contractor is the configuration of the control valve, be it two-way, three-way mixing, three-way diverting, modulating, two-position, etc. This is where you find this critical piece of information, if nowhere else. If indicated elsewhere on the plans, then this serves as verification. If inconsistent with what is found elsewhere, then it’s a matter of seeking out clarification on the issue.

Also frequently found on the coil detail are the locations of temperature sensors. The detail that I’m currently looking at as I write this column shows an immersion sensor on both the inlet and the outlet of the coil. However when I check the points list in the temperature controls specification for this project, I do not find a requirement for coil temperature sensing points. Since this is a single instance on this particular project, I’m accounting for it as it won’t drastically affect my price and scope. Yet if there were dozens of these coils, I may reconsider and ask for clarification before I generate a bid. As you can see, these details are important as well as they are sometimes confusing and contradictory to other information found in the contract documents.

You may find other items such as flow meters, BTU meters, and drainpan detectors shown on the coil details and not found anywhere else. It’s acceptable for these types of devices to be only shown on the details. In fact if that’s the only place they’re indicated, then there’s no inconsistency. If they are indicated elsewhere, then again the coils detail serves to confirm the need.

Pump Details

Staying on the waterside, we consider pump details, those depicting either a base-mounted or inline pump, with the mechanical (piping) trim. Sometimes these details will include flow switch and temperature sensor locations. I’ve found that often these details will be generic in nature, and may or may not be in alignment with what’s called for in the specs. The last time I saw pump status proved by a flow switch was in nineteen ninety whaaat? More recently, I’ve seen pump status called for to be proven by a differential pressure switch across the pump, and I still see details that indicate this. These days we’re inclined to confirm status of any motor, be it a pump or a fan, by using a current-sensing switch. Regardless of what some pump detail may show.

Exhaust Fan Details

Turning to the airside, the first that comes to my mind is the exhaust fan detail. Whether roof-mounted, sidewall, or inline, there are things to look for and be mindful of. With roof-mounted exhaust fans, the detail may show a backdraft damper, whether motorized or gravity. The detail that I’m looking at now does not specify, and so I may need to go elsewhere for this information, like the equipment schedule. Which begs the question, “Who’s responsible for providing the damper”. If it’s a gravity damper then I’ll exclude it, as it is technically not a control device. However if it’s to be a motorized damper, then either I’m to provide it, or it’ll come from the factory, as part of the exhaust fan assembly. Same goes for side-wall fans. Is there a damper? Is it motorized? If so, who’s to provide?

Terminal Unit Details

For VAV and fan-powered boxes, the temperature control points are generally accepted and often included in a points list for these equipment. Still, it’s good practice to review the details, as I’ve found anomalies that I’ve needed to be aware of. Discharge air temperature sensors shown on units with heating coils, for example. No need to provide a discharge air sensor if the VAV box is cooling-only, I’m sure we’ll agree with. But if the unit has an electric or hot water coil, the detail may show this, even if the points list omits it. The other one that creeps up is the requirement for a pressure-independent control valve. This will typically be written up in the spec, yet may also be indicated in the terminal unit detail (or not!).

Miscellaneous Details

Reliable Controls Other details often contain information pertinent to the controls contractor. We’re familiar with the typical boiler and chiller system details, those that are schematic in nature in order to show the design concepts of the hot or chilled water systems that they depict. Included in these details are sensor locations, be it both temperature and pressure. Also shown are control valves that don’t associate with any particular coils, such as system bypass valves and boiler/chiller isolation valves. These control valves are often large in size, so don’t miss these!

Packaged unit details, such as those for rooftop units and packaged make-up air units, may provide some additional insight as to what’s required. For instance, the equipment schedule for the rooftop units may indicate in the notes that a duct-mounted smoke detector is required for all units over 2,000 CFM. The detail may show where that detector is to be located. With any air handler of substantial size, there’s always the possibility of having to furnish and install airflow measuring stations, especially if the project is striving to achieve LEED certification. It is possible that the only place this is shown is on the equipment detail.

The devil is in the details…literally! Ignoring these all-important sheets of the mechanical plans, when procuring a contract for the installation of the temperature control systems, can lead to being awarded the project, only to find out that you’ve underbid because you missed some big-ticket items shown only on the mechanical details!

Tip of the Month: Find the details sheets of any given plans and specifications project, and highlight anything remotely associated with controls. Make a list of these items and keep the list handy. Use the list as a reference of what you should be looking for on the details sheets of future projects. Update the list whenever you find something new. In the long run, this list may just help you procure a project, or at the very least keep you out of trouble!

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